The field of the present invention relates to a process for continuously preparing diaryl carbonates from dialkyl carbonates and at least one monohydroxyl compound in the presence of catalysts, and to the use thereof for preparation of polycarbonates. The alkylene glycol obtained in the preparation of the dialkyl carbonate used is recycled by oxidative carbonylation with carbon monoxide in the presence of a catalyst to give the cyclic alkylene carbonate which is in turn converted to the dialkyl carbonate. More particularly, the process consists in the utilization of the alkylene glycol obtained for the diphenyl carbonate preparation process (DPC process).
It is known that diaryl carbonates, especially diphenyl carbonate, can be obtained by phase interface phosgenation (Schotten-Baumann reaction) of monophenols in an inert solvent in the presence of alkali and a catalyst. The use of solvents and sodium hydroxide solution is disadvantageous, since the aqueous alkali can cause partial hydrolysis of phosgene or chlorocarbonic esters, large amounts of sodium chloride are obtained as a by-product, and the solvent and catalyst have to be recovered.
The preparation of aromatic and aliphatic-aromatic carbonic esters (carbonates) by transesterification proceeding from aliphatic carbonic esters and monophenols is also known in principle. This is an equilibrium reaction wherein the equilibrium position is shifted almost entirely in the direction of the aliphatically substituted carbonates. It is therefore comparatively easy to prepare aliphatic carbonates from aromatic carbonates and alcohols. In order, however, to carry out the reaction in the reverse direction towards aromatic carbonates, it is necessary to shift the very unfavourable equilibrium effectively to the side of the aromatic carbonates, for which not only very active catalysts but also suitable process regimes have to be employed.
The processes known from the literature, for example EP-A 461 274, DE-A 42 26 755, DE-A 42 26 756, however, generally describe only those process steps in which the reaction to give the diaryl carbonate takes place by transesterification and/or disproportionation. WO-A 2006/033291, EP-A 1 775 280, EP-A 1 767 516, EP-A 1 767 517, EP-A 767 518, EP-A 1 762 559 and EP-A 1 762 560 additionally give hints with regard to the apparatus configurations of reaction columns for preparation of diaryl carbonates. For the economic viability of a process, however, not just the process sections in the region of the reaction but, in some cases to a much greater degree, the subsequent steps for workup are of relevance.
Since the preparation of diaryl carbonates by reaction of an aromatic hydroxyl compound with a dialkyl carbonate, as experience has shown, is energetically very demanding, measures for reducing the energy consumption likewise play an important role. In this regard, there are also some approaches in the literature to energetic and apparatus integration.
The preparation of dialkyl carbonates by transesterifying cyclic alkylene carbonate and alkyl alcohol is known and has been described many times. U.S. Pat. No. 6,930,195 B described this catalysed transesterification reaction as a two-stage equilibrium reaction. In the first reaction stage, the cyclic alkylene carbonate reacts with alkyl alcohol to give hydroxyalkyl alkyl carbonate as an intermediate. The intermediate is then converted in the second reaction stage with the aid of alkyl alcohol to give dialkyl carbonate and alkylene glycol.
For the industrial implementation of the dialkyl carbonate preparation process, the use of a reactive distillation column (also referred to hereinafter as transesterification column), which has already been described in documents including EP 530 615 A, EP 569 812 A and EP 1 086 940 A, has been found to be particularly favourable. In EP 569 812 A, the cyclic alkylene carbonate is introduced continuously into the upper part of the transesterification column, and the dialkyl carbonate-containing alkyl alcohol into the middle or lower part of the transesterification column. In addition, below the introduction of the dialkyl carbonate-containing alkyl alcohol, virtually pure alkyl alcohol is introduced. The high boiler mixture, which includes the alkylene glycol prepared as a by-product, is drawn off continuously at the bottom of the transesterification column. The low boiler mixture, which comprises the dialkyl carbonate prepared, is drawn off at the top of the transesterification column as dialkyl carbonate-alkyl alcohol mixture and subjected to a further purification step.
A disadvantage is the formation of the alkylene glycol as a coproduct, which can be used for the preparation of polyesters only with good optical properties (fibre quality). In order to achieve the good optical properties, it is therefore necessary to drive high expenditure for purification of the alkylene glycol, which adversely affects the economic viability of the process.
Therefore, the reaction of alkylene glycol with urea to give cyclic alkylene carbonate and ammonia has also already been described in EP 638 541 B1. A disadvantage here is that ammonia does not find use for polycarbonate preparation and the conversion of ammonia and carbon dioxide to urea is barely economically viable, if at all.
It is known that the oxidative carbonylation of monoalcohols with carbon monoxide and oxygen in the presence of Co catalysts and Cu catalysts such as CuCl leads to dialkyl carbonate, as described in EP 463 678 A2 or EP 413 217 A2. A disadvantage is the corrosive properties of the catalyst system.
U.S. Pat. No. 4,131,521 describes the electrochemical oxidation of dialcohols such as ethylene glycol to give ethylene carbonate. A disadvantage is the formation of hydrogen as a by-product, which does not find use in polycarbonate preparation.
Tetrahedron Letters 50, 7330 (2009) describes the reaction of alkylene glycols with carbon monoxide and oxygen with PdI2/KI as a catalyst system. Disadvantages are the low turnover number (TON) and the low selectivity of 84%.
EP 781 760 A1 describes a continuous process for preparing aromatic carbonates by reacting a dialkyl carbonate with an aromatic hydroxyl compound in the presence of a catalyst and continuously removing the aromatic carbonate formed in the reaction, the alcoholic by-products, the dialkyl carbonate and the aromatic hydroxyl compound, the dialkyl carbonate and the aromatic hydroxyl compound being recycled into the reaction.
EP 1 638 917 A1 describes a process for recovering a product from a waste stream by contacting with an alkyl alcohol, the product recovered comprising diaryl carbonate, aromatic alcohol, alkyl salicylate and alkyl alcohol. One disadvantage of the process described is that the reaction is effected in three stages, which makes it very complicated. Another is that high-boiling waste streams are obtained at two points. Removal of the catalyst before the isolation of the diaryl carbonate gives rise to the first waste stream, and the subsequent workup consisting of two distillation columns to the second waste stream. The workup for isolation of the diaryl carbonate is thus very demanding both in apparatus and energetic terms. In addition, the quality of the diaryl carbonate thus prepared at 99.5% by weight is very poor and it is unsuitable for the preparation of polycarbonate. The separation of the mixture of alcohol of reaction and dialkyl carbonate obtained in the reaction is not described either.
WO-A 2005/000776 describes a process for preparing an alkyl aryl ether which is formed in the reaction of a dialkyl carbonate with an aromatic hydroxyl compound. In this process, diaryl carbonate is additionally also obtained. The process structure comprises three reaction columns and two further distillation columns for the purpose of isolating the alkyl aryl ether. The fact that a controlled purification of the alkyl aryl ether is an aim in the process described here leads to the conclusion that the amount formed in the reaction is high. In the preparation of diaryl carbonates, however, the recovery of a high-purity alkyl aryl ether is not first priority, and the aim is instead minimum formation of this by-product obtained in the transesterification. Moreover, the reaction regime comprising three reaction stages is very complicated, and no information is given with regard to the workup of the diaryl carbonate and the separation of the mixture which is obtained in the reaction and comprises dialkyl carbonate and alcohol of reaction. EP-A 1 237 842 A1 also describes a comparable process, and therefore the disadvantages already mentioned likewise apply to this.
WO-A 2004/016577 describes a process for preparing aromatic carbonates from dialkyl carbonate and an aromatic hydroxyl compound in the presence of a catalyst in a plurality of separate and series-connected reaction zones of a reactor arrangement, wherein the heat of condensation obtained in the condensation of the vapour stream of the last reaction zone is used to heat the liquid stream introduced into the first reaction zone. However, a disadvantage of this process is the complicated reactor arrangement. In addition, the energetic integration of this process is in need of improvement and is limited only to the process section of reaction. Subsequent steps for the workup are not described.
JP-A 2002-020351 describes a batchwise process for preparing diaryl carbonate, from which heat can be utilized for steam raising. However, disadvantages of this process are the batchwise performance and the reactor arrangement used for the reaction, which has an attached distillation column. Subsequent steps for the workup are not described.
The chemical oxidation of alcohols with carbon monoxide and oxygen to give alkyl carbonates is known in principle.
For instance, EP 463 678 A2 describes the oxidative carbonylation of alcohols with carbon monoxide and oxygen in the presence of Co catalysts and Cu catalysts such as CuCl to give dialkyl carbonate.
The oxidative carbonylation of dialcohols with carbon monoxide and oxygen to give alkylene carbonate is also known.
U.S. Pat. No. 4,131,521 describes the formation of ethylene carbonate in 5 to 10% yield by electrochemical oxidation of ethylene glycol in the presence of NH4Br. Disadvantages are the low economic viability of an electrochemical oxidation and the formation of hydrogen as a by-product.
DE A 22 22 488 describes a process for preparing cyclic glycol carbonates from glycols with CO/O2 in the presence of copper ions. Disadvantages are the short service life of the catalyst with a TON below 93 and the low activity of the catalyst with a turnover frequency below 30 molglycol·molCu−1·h−1.
Organometallics, 25, 2872 (2006) describes the preparation of ethylene carbonate from ethylene glycol. A disadvantage is that the expensive Pd catalyst is required in stoichiometric amounts.
Tetrahedron Lett. 50, 7330 (2009) describes the oxidative carbonylation of dialcohols such as ethylene glycol to give ethylene carbonate with PdI2/KI as a catalyst formulation. In spite of a great CO excess, only a moderate TON is mentioned.
J. Org. Chem. 51. 2977 (1986) describes the conversion of ethylene glycol and 1-phenylethanediol to the corresponding cyclic carbonates. A disadvantage is the use of metal salts as an oxidant in stoichiometric amounts.
There was accordingly still a need to provide a process for preparing aromatic carbonates, preferably diaryl carbonates, which includes recycling of the alkylene glycol by-product formed, which does not have the disadvantages specified above and in which, compared to the known processes specified above, avoidance of the coproduct is possible or can be achieved in an efficient manner.
The problem underlying the invention was accordingly to provide a process for preparing aromatic carbonates, preferably diaryl carbonates, which comprises preparation of the dialkyl carbonate used with recycling of the alkylene glycol obtained as a coproduct.